1. Field of the Invention
This invention relates to carbonless materials. More particularly it relates to colorless pressure sensitive layers on substrates. Many existing compositions exhibit a yellow or brown color cast which is caused by the color of the reactive metal compounds contained therein. This invention uses compositions containing colorless iron salts which are reactable at room temperature to give a visible image.
In commercial applications, pressure sensitive labels are sought which not only provide visible images but which are also capable of being read by optical scanners using near infrared radiation (NIR). The images resulting from reacting the colorless iron salts with chelates having certain substituents exhibit good discrimination both visually and to NIR.
2. Background of the Art
For many years heat and pressure sensitive imaging sheets have been used for copying and labeling. Many of these materials involve the mixing of two or more physically separated reagents to cause a color forming reaction. Several general classes of color forming reactants have been used, of which two common ones are (a) leuco lactone or spiropyran compounds reactable with phenolic compounds (e.g. U.S. Pat. Nos. 3,829,401 and 3,846,153) and (b) heavy metal salts of organic acids reactable with ligands to give colored complexes (e.g. U.S. Pat. Nos. 2,663,654, 3,094,620, 3,293,055, 3,953,659, 4,334,015, 4,513,302, 4,531,141, 4,533,930 and 4,602,264).
Commercial preference for the heavy metal salt class has often resulted from the high stability and near black color of the images produced (U.S. Pat. No. 4,531,141). Of the heavy metals used, iron, nickel, and cobalt are common and ferric iron appears to be preferred (U.S. Pat. Nos. 2,663,654, 3,953,659, 4,531,141, 4,533,930 and 4,602,264).
The objection raised to the ferric salt phenolic ligand systems is the colored nature of the unreacted ferric salt. This has led to the use of white fillers (U.S. Pat. No. 4,531,141) or other incident light scattering devices (e.g., "blushing" the surface of the layer as in U.S. Pat. No. 3,953,659) to reduce the observed color tint of the coated layer.
Recently, there has been interest in obtaining reactive iron salts which are colorless and which give sharp, high density images when reacted with a colorless ligand. Organophosphates of ferric iron are known in the art to be amongst the few colorless ferric salts (Smythe et al., J. Inorg. Nucl. Chem., 30 1553-1561, (1968)). In U.S. Pat. Nos. 4,533,930 and 4,602,264 it is disclosed that such organophosphates, and the equivalent thiophosphates, can react with a variety of ligands under the influence of heat or pressure to give colored results. Ferric salts of organophosphinic acids and organophosphonic acids are included in those disclosures. Some of these organophosphates and many of the thiophosphates have some color cast before reaction which appears to be obscured by the use of white filler in the compositions. In these two patents there are disclosed pressure sensitive manifold papers in which at least one of the two reactants is encapsulated as a solvent solution. When the microcapsules are burst by pressure, the reactants come into contact and immediately react at room temperature to give a colored result. These patents further disclose the use of ferric organophosphates containing organic acid moieties formed by the aqueous reaction of a ferric salt, an alkali metal organophosphate, and an alkali metal salt of an organic acid. These are disclosed as giving the initial material better "color forming properties" and giving better image colors (U.S. Pat. No. 4,533,930, Column 5, lines 38-39 and U.S. Pat. No. 4,602,264, Column 5, lines 7-9) than the simple organophosphates. Excess organic acid salt is disclosed as degrading the white color. It is of significance that the inventors do not consider the choice of the ferric salt used in the preparation to be important. In fact they specifically mention ferric chloride and ferric sulfate (U.S. Pat. No. 4,533,930, Column 6, lines 10-17 and U.S. Pat. No. 4,602,264, Column 6, lines 12-18) and all of their examples use ferric chloride.